This invention relates to apparatus for protecting articles against unauthorized removal.
More particularly, it relates to apparatus which is used to electronically detect articles which are being removed from a store or other protected premises. A particular application of such apparatus is in the detection of shoplifting.
In one known form, such apparatus includes means for generating, in the exit path from the protected premises, a radio-frequency field which is periodically swept through a predetermined frequency range. Attached to the protected articles are tags bearing inductance-capacitance circuits which are resonant within the swept-frequency range. When an article with such a tag attached is carried out through the exit (and therefore through the radio-frequency field), it disturbs that field in a manner which can be sensed by a detector. This detector provides an output which can be used to operate an indicator, e.g. an alarm such as a buzzer or a light.
Apparatus of this type is disclosed, for exampled, in U.S. Pat. No. 3,810,147, issued May 7, 1974, the contents of which are incorporated herein by reference as though fully set forth herein.
In order to distinguish between articles whose removal is authorized (e.g. by having been paid for prior to store exiting) and those whose removal is not authorized (because they are being shoplifted), it is known to apply to the tags on the former a separate adhesive label which incorporates a layer of electrically conductive material such as metal foil. Such a label has the effect of disabling the capability of the resonant circuit to disturb the electric field. Therefore, an article which carries both the resonant circuit-bearing tag and the superposed adhesive label will not cause an alarm, whereas one which has not been provided with such a label will cause an alarm.
In practice, when dealing with shoplifting prevention, the foil-bearing label will typically be applied by the store clerk who performs the check-out operation, at the time when payment is made for the article which is to be carried out of the store.
A problem which has sometimes arisen in connection with apparatus of the type described above is that the application of the foil-bearing label does not always provide a totally reliable way to prevent the resonant circuit-bearing tag from causing an alarm. The reason is that such application is typically only one of several functions which have to be performed by the store clerk, who is often under the pressure of a large volume of customers and goods to be processed. Therefore, the application of such a label may not be carried out meticulously enough to cause permanent adhesion of the label to the tag. Impermanent adhesion would create the risk that the label would sooner or later fall off, or be stripped of by handling of the merchandise, whereupon the resonant circuit-bearing tag would become unintentionally capable of again causing an alarm.
Because of this, the striving has been to enhance the adhesion between the foil-bearing label and the circuit-bearing tag, so that even relatively careless application would nevertheless result in a combination of tag and label which are permanently adhered to each other.
While quite successful in achieving its intended purpose, this enhancement of adhesion permanence between label and tag also had some undesired side-effects. In particular, it reduced the flexibility of utilization of the overall article protection technique. It is not always the case that a resonant circuit-bearing tag, once made alarm-ineffective by application of a foil-bearing label, should remain permanently ineffective. For example, when the articles to be protected are video tape cassettes which are distributed through rental rather than sale, it is desired to have the protection go through cycles of effectiveness followed by ineffectiveness. While a cassette is in the store, before it has been checked out for rental, the circuit-bearing tag which is attached to it should be effective to cause an alarm if the cassette is carried out through the protected store exit. In contrast, when the cassette has been properly checked out, the tag should become ineffective, so that no alarm is given when the checked-out cassette leaves the store. This condition should continue to prevail until the cassette is carried back into the store and is formally checked back in. After that, however, the cassette's protection by an alarm-effective tag should be restored. This cycle then should be repeatable during successive rentals.
Use of the conventional foil-bearing label in this situation meant that each time a cassetts was returned, a new circuit-bearing tag had to be applied in order to reestablish the desired alarm effectiveness.
This was not only costly, but also created a problem in view of the limited space which is available on the cassette itself for the placement of such tags.